pw_varint/public/pw_varint/varint.h

// Copyright 2020 The Pigweed Authors
//
// Licensed under the Apache License, Version 2.0 (the "License"); you may not
// use this file except in compliance with the License. You may obtain a copy of
// the License at
//
//     https://www.apache.org/licenses/LICENSE-2.0
//
// Unless required by applicable law or agreed to in writing, software
// distributed under the License is distributed on an "AS IS" BASIS, WITHOUT
// WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See the
// License for the specific language governing permissions and limitations under
// the License.
#pragma once

#include <stddef.h>
#include <stdint.h>

#include "pw_preprocessor/compiler.h"

#ifdef __cplusplus
extern "C" {
#endif

// Expose a subset of the varint API for use in C code.

size_t pw_VarintEncode(uint64_t integer, void* output, size_t output_size);
size_t pw_VarintZigZagEncode(int64_t integer, void* output, size_t output_size);

size_t pw_VarintDecode(const void* input, size_t input_size, uint64_t* output);
size_t pw_VarintZigZagDecode(const void* input,
                             size_t input_size,
                             int64_t* output);

// Returns the size of an when encoded as a varint.
size_t pw_VarintEncodedSize(uint64_t integer);
size_t pw_VarintZigZagEncodedSize(int64_t integer);

#ifdef __cplusplus

}  // extern "C"

#include <span>
#include <type_traits>

#include "pw_polyfill/language_feature_macros.h"

namespace pw {
namespace varint {

// The maximum number of bytes occupied by an encoded varint.
PW_INLINE_VARIABLE constexpr size_t kMaxVarint32SizeBytes = 5;
PW_INLINE_VARIABLE constexpr size_t kMaxVarint64SizeBytes = 10;

// ZigZag encodes a signed integer. This maps small negative numbers to small,
// unsigned positive numbers, which improves their density for LEB128 encoding.
//
// ZigZag encoding works by moving the sign bit from the most-significant bit to
// the least-significant bit. For the signed k-bit integer n, the formula is
//
//   (n << 1) ^ (n >> (k - 1))
//
// See the following for a description of ZigZag encoding:
//   https://developers.google.com/protocol-buffers/docs/encoding#types
template <typename T>
constexpr std::make_unsigned_t<T> ZigZagEncode(T n) {
  static_assert(std::is_signed<T>(), "Zig-zag encoding is for signed integers");
  using U = std::make_unsigned_t<T>;
  return (static_cast<U>(n) << 1) ^ static_cast<U>(n >> (sizeof(T) * 8 - 1));
}

// ZigZag decodes a signed integer.
// The calculation is done modulo std::numeric_limits<T>::max()+1, so the
// unsigned integer overflows are intentional.
template <typename T>
constexpr std::make_signed_t<T> ZigZagDecode(T n)
    PW_NO_SANITIZE("unsigned-integer-overflow") {
  static_assert(std::is_unsigned<T>(),
                "Zig-zag decoding is for unsigned integers");
  return static_cast<std::make_signed_t<T>>((n >> 1) ^ (~(n & 1) + 1));
}

// Encodes a uint64_t with Little-Endian Base 128 (LEB128) encoding.
inline size_t EncodeLittleEndianBase128(uint64_t integer,
                                        const std::span<std::byte>& output) {
  return pw_VarintEncode(integer, output.data(), output.size());
}

// Encodes the provided integer using a variable-length encoding and returns the
// number of bytes written.
//
// The encoding is the same as used in protocol buffers. Signed integers are
// ZigZag encoded to remove leading 1s from small negative numbers, then the
// resulting number is encoded as Little Endian Base 128 (LEB128). Unsigned
// integers are encoded directly as LEB128.
//
// Returns the number of bytes written or 0 if the result didn't fit in the
// encoding buffer.
template <typename T>
size_t Encode(T integer, const std::span<std::byte>& output) {
  if (std::is_signed<T>()) {
    return pw_VarintZigZagEncode(integer, output.data(), output.size());
  } else {
    return pw_VarintEncode(integer, output.data(), output.size());
  }
}

// Decodes a varint-encoded value. If reading into a signed integer, the value
// is ZigZag decoded.
//
// Returns the number of bytes read from the input if successful. Returns zero
// if the result does not fit in a int64_t / uint64_t or if the input is
// exhausted before the number terminates. Reads a maximum of 10 bytes.
//
// The following example decodes multiple varints from a buffer:
//
//   while (!data.empty()) {
//     int64_t value;
//     size_t bytes = Decode(data, &value);
//
//     if (bytes == 0u) {
//       return Status::DataLoss();
//     }
//     results.push_back(value);
//     data = data.subspan(bytes)
//   }
//
inline size_t Decode(const std::span<const std::byte>& input, int64_t* value) {
  return pw_VarintZigZagDecode(input.data(), input.size(), value);
}

inline size_t Decode(const std::span<const std::byte>& input, uint64_t* value) {
  return pw_VarintDecode(input.data(), input.size(), value);
}

// Returns a size of an integer when encoded as a varint.
constexpr size_t EncodedSize(uint64_t integer) {
  return integer == 0 ? 1 : (64 - __builtin_clzll(integer) + 6) / 7;
}

// Returns a size of an signed integer when ZigZag encoded as a varint.
constexpr size_t ZigZagEncodedSize(int64_t integer) {
  return EncodedSize(ZigZagEncode(integer));
}

}  // namespace varint
}  // namespace pw

#endif  // __cplusplus